This invention relates to battery charging circuits and, in particular, to circuits suitable for use with power tools which include a self-contained battery pack. In recent times there has been a trend towards making common power tools, such as electric drills, grass clippers and the like, relatively independent of a permanent source of power. This has been accomplished by providing such tools with self-contained rechargeable batteries which can provide energy to the tool for a satisfactory period of time before recharging of the batteries is required.
Typically, the batteries used for such application are of the nickel cadmium type and are suitable for recharging at a split rate. That is, the battery can be recharged at a fast rate as long as this charge rate is discontinued when the full charge condition is reached. Thereafter, only a trickle charge may be applied if damage to the battery is to be avoided. The trickle charge will maintain the battery in a full charge condition for an indefinite period of time. Failure to properly switch from fast charge to trickle charge will quickly cause damage to the battery cells requiring their replacement.
Accordingly, with the increase in popularity of the battery powered tools there has developed a need for a low cost yet highly reliable battery charging circuit which can be included with each tool for safely recharging the tool batteries.
Battery charging circuits for this purpose are known; in particular, see the patents referenced in the prior art statement which follows. Such circuits are generally more expensive to manufacture than the present invention and many of them do not have the capability of preventing reversion to the fast charge mode after the batteries have become fully charged.
It is accordingly an object of the present invention to provide a low cost battery charging circuit for rechargeable batteries of the type suitable for use in portable power tools.
It is a further object of the invention to provide a battery charging circuit capable of fast and trickle charge operating modes.
It is a further object of the invention to provide a battery charging circuit as specified in the preceding object which includes means for preventing reversion to the fast charge mode after a battery has become fully charged.
Another object of the invention is to provide a battery charging circuit capable of recharging a battery in a fast charge mode and maintaining it at full charge by applying a trickle charge thereafter.
Other objects and advantages of the invention will be apparent from the remaining portion of the specification.